Gamification--the use of game mechanics in conventionally nongame activities--has received attention in the field of education. Games, however, are not reducible to the common mechanisms of gamification that target extrinsic motivation, and may also include elements such as role playing, world making, and collective storytelling. Here, the authors discuss the potential learning benefits of large-scale and situated alternate reality games (ARGs) that complicate conventional gamified systems. They also explore the scaling up of improvisational modes of play in these games from intimate groups to large collectives exceeding the size of typical classrooms. They use a case study of "The Source" (2013), an ARG they designed (with funding from the National Science Foundation) for urban youth of color from the South Side of Chicago in an out-of-school setting using play across several platforms. "The Source" aimed to promote the academic areas of STEM (science, technology, engineering, and mathematics), as well as twenty-first-century literacies and social justice. The authors argue that such ARGs facilitate learning by engaging semifictional and immersive play made flexible and extensible through game forms. They suggest that, although designers determine the challenges in an ARG, the players shape the experience and shared game world through collaborative actions.